The case is extremely troublesome because respondents have made such a strong showing that they will suffer irreparable harm if denied the use of marijuana to treat their serious medical illness.But the question before us is not whether marijuana does in fact have valid therapeutic purposes, nor whether it is a good policy for the Federal Government to enforce the Controlled Substances Act in these circumstances.Rather, the only question before us is whether Congress has the power to prohibit respondents’ activities.California law does not really affect our answer to that question for it is well-settled that the outer limits of congressional power under the Commerce Clause are defined exclusively by federal law.The Supremacy Clause unambiguously provides that if there is any conflict between federal and state law, federal law shall prevail.